1. Field of the Invention
The invention relates to the preparation of aromatic amines which are substituted by C.sub.1 -C.sub.4 -alkoxy in the p-position by catalytic hydrogenation of the underlying aromatic nitro compounds in a reaction medium comprising sulphuric acid and a C.sub.1 -C.sub.4 -alcohol in the so-called Bamberger type rearrangement.
In the Bamberger rearrangement, p-methoxy-aniline is formed, for example, from phenylhydroxylamine under the action of mineral acids and addition of methanol as the nucleophile. As can be seen from the equation below, it is also possible for water to occupy the p-position as the nucleophile with the formation of p-amino-phenol; furthermore, the nucleophilecanalso occupy the o-position with respect to the original hydroxylamine group. For a long time, this Bamberger rearrangement has been associated with the intermediate formation of the hydroxylamine from the underlying nitro compound by catalytic hydrogenation or electrochemical reduction. The mineral acid described most often for this purpose and the only suitable one for industrial applications is sulphuric acid. In such a combination of the hydrogenation of the nitro group with the Bamberger rearrangement, the formation of the not further substituted amino compound from the underlying nitro compound can take place as a further side reaction. These processes can be summarised by the following equations, which exemplify the reduction and reaction of nitrobenzene: ##STR1##
p-Methoxy-aniline (a), which is the product formed in the largest amount in sulphuric acid/methanolic environment, is the generally desired product. In addition, p-aminophenol (b), o-methoxy-aniline (c) and aniline (d) are formed as by-products. The latter (d) is apparently formed here by competition with the Bamberger compound by further reduction of the phenylhydroxylamine.
A further undesired side reaction which takes place is ether formation from the alcohol used, especially since the Bamberger rearrangement which is coupled with the reduction of the nitro compound represents a reaction lasting several hours and even more so, since the reaction is in general carried out at elevated temperature in order to avoid an even longer reaction.
2. Description of the Related Art
It is known that nitroaromatics, such as, for example, nitrobenzene, 2- and 3-nitro-toluene, 2,3-dinitro-toluene or 6-chloro-2-nitrotoluene, can be reduced electrochemically in methanol and in the presence of sulphuric acid or methylsulphuric acid to give 4-alkoxy-aminoaromatics (see German Offenlegungsschrift 2,617,808, JP 55/154,590, Kagaku Kogyo 56 293-296 (1982)). Processes of this type cause high costs of energy and a considerable investment in apparatus.
Furthermore, it is known that nitroaromatics, such as, for example, 2-chloronitrobenzene, 2-nitro-toluene or 2,6-dimethyl-nitrobenzene, can be reacted in alcoholic solution with addition of water or carboxylic acids at a hydrogen pressure of between 0.01 and 1 bar to give 4-alkoxy-aminoaromatics (see German Offenlegungsschrift 3,443,385 and JP 61/109,759). As is known from the literature, the undesirable formation of p-hydroxyaminoaromatics must be expected in the presence of water in a type of competing conventional Bamberger reaction in aqueous reaction media (see HOUBEN-WEYL VI/lc, 91-92). The addition of carboxylic acids is economically a disadvantage.
It is furthermore known that various nitroaromatics, such as, for example, nitrobenzene, can be reacted in alcoholic solution and in the presence of sulphuric acid with the addition of catalyst poisons, such as, for example, dimethyl sulphoxide, or with the use of modified and deactivated catalysts, such as platinum oxide or platinum sulphide, at pressures of up to 6 bar of hydrogen to give 4-alkoxy-aminoaromatics (SU523,082, SU 520,347, SU 514,811 and SU 578,302 and Nippon Kagaku Kaishi 1237 (1982), 245 (1980) and 1532 (1979)). Modified catalysts of this type are expensive and in most cases are not or only to a very limited extent reusable, which constitutes a significant economic disadvantage. The addition of moderators or inhibitors, such as dimethyl sulphoxide, moreover makes the workup and purification of the crude reaction products more difficult and results in lengthening of the reaction times, due to deactivation of the noble metal catalysts, and thus in unfavourable space-time yields.
According to the literature, when Bamberger reactions are carried out in alcoholic reaction media for the purpose of preparing p-alkoxy-aminoaromatics, it must be expected that, if higher hydrogen pressures than about 0.2-2 bar of hydrogen pressure are used, the aminoaromatics formed by simple reduction of the nitro groups or even ringhydrogenated compounds are predominantly obtained. In order to prevent this, elevated pressures are in general avoided and in some cases substantially lengthened reaction times are accepted (see German Offenlegungsschrift 3,443,385, JP 53/084,925, 57/072,945 and 57/002,247).